Re: Kernel BUG on Snapshot Deletion (3.11.0-rc5)

[Mitch Harder <mitch.harder@sabayonlinux.org>]

On Fri, Aug 23, 2013 at 3:48 AM, Stefan Behrens
<sbehrens@giantdisaster.de> wrote:
> On Wed, 21 Aug 2013 08:44:55 -0500, Mitch Harder wrote:
>> I've had a hard time assembling a portable reproducer for this issue.
>>
>> I discovered that my reproducer was highly dependent on a local
>> archive of out-of-date git kernel sources.  My efforts to reproduce
>> the error with a portable set of scripts with publicly available
>> kernel git sources weren't successful.
>>
>> It seems like this issue is related to a corner-case workload that is
>> difficult to reproduce.
>>
>> So I've bisected the error I was seeing with my local script, and
>> identified the following commit as triggering my issue:
>>
>> commit:    3c64a1aba7cfcb04f79e76f859b3d66660275d59
>> Btrfs: cleanup: don't check the same thing twice
>> https://git.kernel.org/cgit/linux/kernel/git/mason/linux-btrfs.git/commit/fs/btrfs?h=for-linus&id=3c64a1aba7cfcb04
>>
>> I tested a kernel which reverted this change, and also added WARN_ON
>> lines to provide a back trace.
> [...]
>> diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
>> index cd46e2c..a1091f7 100644
>> --- a/fs/btrfs/inode.c
>> +++ b/fs/btrfs/inode.c
>> @@ -2302,6 +2302,12 @@ static noinline int
>> relink_extent_backref(struct btrfs_path *path,
>>              return 0;
>>          return PTR_ERR(root);
>>      }
>> +    if (btrfs_root_refs(&root->root_item) == 0) {
>> +        srcu_read_unlock(&fs_info->subvol_srcu, index);
>> +        /* parse ENOENT to 0 */
>> +        WARN_ON(1);
>> +        return 0;
>> +    }
> [...]
>> [ 1616.886868] ------------[ cut here ]------------
>> [ 1616.886912] WARNING: at fs/btrfs/inode.c:2308 relink_extent_backref+0x103/0x721 [btrfs]()
>> [ 1616.887050] Call Trace:
>> [ 1616.887064] [<ffffffff8161a34a>] dump_stack+0x19/0x1b
>> [ 1616.887071] [<ffffffff8103035a>] warn_slowpath_common+0x67/0x80
>> [ 1616.887077] [<ffffffff8103038d>] warn_slowpath_null+0x1a/0x1c
>> [ 1616.887100] [<ffffffffa019ea82>] relink_extent_backref+0x103/0x721
>> [ 1616.887205] [<ffffffffa019f7e2>] btrfs_finish_ordered_io+0x742/0x829
>
> Mitch,
>
> Thank you for this excellent work to find the cause of the issue. I've sent a patch "Btrfs: fix for patch "cleanup: don't check the same thing twice"" and would appreciate if you could repeat your test, just to make sure, because I was never able to reproduce this issue myself.
>

Thanks.

I've tested my "special" workload with your patch on the latest
3.11_rc6 kernel, and the patch corrects the errors I was encountering.